Logging while drilling tool for obtaining azimuthally sensitive formation resistivity measurements

ABSTRACT

An apparatus for making azimuthally sensitive resistivity measurements of a subterranean formation is disclosed. The apparatus includes a magnetically permeably ring deployed about an electrically conductive tool body. An AC voltage supply is coupled to the tool body on opposing sides of the magnetically permeable ring, with at least one connecting conductor crossing outside the ring. Exemplary embodiments of this invention may further include one or more current sensing electrodes deployed in and electrically isolated from an outer surface of a blade deployed on the tool body and may be utilized to make azimuthally resolved formation resistivity measurements

RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.11/080,777, which was filed Mar. 15, 2005.

FIELD OF THE INVENTION

The present invention relates generally to an apparatus for logging asubterranean borehole. More specifically, this invention relates to anapparatus, such as a logging while drilling tool, for making azimuthallysensitive resistivity measurements of a subterranean formation.

BACKGROUND OF THE INVENTION

The use of electrical measurements in prior art downhole applications,such as logging while drilling (LWD), measurement while drilling (MWD),and wireline logging applications is well known. One such electricalmeasurement technique is utilized to determine a subterranean formationresistivity, which, along with formation porosity measurements, is oftenused to indicate the presence of hydrocarbons in the formation. Forexample, it is known in the art that porous formations having a highelectrical resistivity often contain hydrocarbons, such as crude oil,while porous formations having a low electrical resistivity are oftenwater saturated. It will be appreciated that the terms resistivity andconductivity are often used interchangeably in the art. Any referencesto the determination or use of resistivity in this application areintended to generically include conductivity as well. Those of ordinaryskill in the art will readily recognize that these quantities arereciprocals and that one may be converted to the other via simplemathematical calculations. Mention of one or the other herein is forconvenience of description, and is not intended in a limiting sense.

Prior art logging while drilling tools utilized to measure formationresistivity, typically utilize one or more wound toroidal core antennas(also referred to as toroidal transmitters and toroidal receivers)deployed in an insulating media along the exterior of the drill collar.As generally described in the prior art, the wound toroidal core antennainduces an electrical current in the drill collar. The electricalcurrent enters the formation on one side of the toroidal transmitter andreturns to the drill collar on the other side of the toroidaltransmitter. Measurement of the current enables a formation resistivityto be determined.

For example, Redwine et al., in U.S. Pat. No. 3,408,561, disclose an LWDapparatus in which a toroidal receiver is deployed about a drill collarnear the drill bit and a toroidal transmitter is deployed about thedrill collar uphole of the toroidal receiver. In use, the voltageinduced in the toroidal receiver provides an indication of theresistivity of the formation. Aarps, in U.S. Pat. No. 3,305,771,discloses a similar apparatus, but including a pair of toroidaltransmitters and a pair of toroidal receivers.

Clark et al., in U.S. Pat. No. 5,235,285, disclose a technique intendedto provide vertically and azimuthally resolved resistivity at multipledepths of investigation. An LWD tool including a tubular drill collarhaving longitudinally spaced first and second wound toroidal coreantennas is utilized. The upper antenna is configured as a transmitterwhile the lower antenna is configured as a receiver. The tool furtherincludes three longitudinally spaced button electrodes deployed in thedrill collar between the wound toroidal core antennas. The buttonelectrodes are intended to provide a return path for electrical currentflow from the formation to the drill collar, with the current in thebutton electrodes being measured to determine a lateral resistivity ofthe regions of the formation opposing the electrodes. The longitudinalspacing of the button electrodes is intended to provide verticallyresolved resistivity at multiple depths of investigation. Clark et al.further disclose rotating the drill collar to obtain azimuthallyresolved resistivity.

The above described prior art resistivity tools are similar in that eachincludes two or more wound toroidal core antennas (one configured as atransmitter and the other configured as a receiver) deployed about adrill collar. These antennas create inductive impendences along theotherwise highly conductive drill collar. It is also known in the art touse such inductance to impede the unwanted flow of electrical currentinto other sections of the drill string or bottom hole assembly. Forexample, in one such device, magnetically permeable rings are deployedabout an electrically conductive drill collar. The rings are positionedbelow a resistivity tool having wound toroidal antennas, and thusincrease the electrical impedance between the resistivity tool and theadjacent bottom hole assembly. A protective, fiberglass sleeve may bedeployed around the magnetically permeable rings to reduce the risk ofmechanical damage to the rings. This type of device is sometimesreferred to as an inductive choke.

While prior art LWD resistivity tools have been used successfully incommercial drilling applications, utilization of a multiple turntoroidal transformer is often problematic. A typical wound toroidal coreantenna has a primary winding including many turns of insulated wiringabout a toroidal core. Construction and protection of the relativelylarge toroidal core (e.g., typically having a diameter in the range of 4to 10 inches) and winding are problematic, especially for use in thedemanding downhole environment associated with geophysical drilling.Wound toroidal core antennas utilized in drilling applications aresubject to high temperatures (e.g., as high as 200 degrees C.) andpressures (e.g., as high as 15,000 psi) as well as various (oftensevere) mechanical forces, including, for example, shocks and vibrationsup to about 650 G per millisecond. Mechanical abrasion from cuttings inthe drilling fluid and direct hits on the antenna (e.g., from drillstring collisions with the borehole wall) have been known to damagewound toroidal core antennas. Moreover, it is typically expensive tofabricate and maintain wound toroidal core antennas capable ofwithstanding the above described downhole environment.

Therefore, there exists a need for an improved apparatus for makingazimuthally sensitive resistivity measurements of a subterraneanformation. In particular, an apparatus not requiring a wound toroidalcore antenna may be potentially advantageous for making such azimuthallysensitive resistivity measurements in LWD applications.

SUMMARY OF THE INVENTION

The present invention addresses one or more of the above-describeddrawbacks of prior art techniques for making azimuthally sensitiveresistivity measurements of a subterranean formation. Embodiments ofthis invention include at least one magnetically permeable ring deployedabout an electrically conductive tool body. The tool body is configuredfor coupling with a drill string. An AC voltage supply is coupled to thetool body on opposing sides of the magnetically permeable ring, with atleast one connecting conductor crossing outside the ring. Themagnetically permeable ring decreases the admittance of the tool body(i.e., increases the resistance to flow of alternating current) suchthat an AC voltage difference may be sustained between the opposingsides of the tool body. Exemplary embodiments of this invention mayfurther include one or more current sensing electrodes deployed in andelectrically isolated from an outer surface of a blade deployed on thetool body. In such exemplary embodiments, azimuthally sensitiveformation resistivity may be determined via measurement of the ACcurrent in the electrode(s). Rotation of the tool in the borehole andmeasurement of the azimuth via a conventional azimuth sensor enables oneto determine the azimuthal variation of formation resistivity.

Exemplary embodiments of the present invention may advantageouslyprovide several technical advantages. For example, embodiments of thisinvention do not require the use of a toroidal transmitter or a toroidalreceiver deployed about the tool body. Rather, the combination of the ACvoltage supply coupled directly to the tool body and the magneticallypermeable ring(s) function as a transmitter. As such, exemplaryembodiments of this invention may provide for improved reliability atreduced costs as compared to prior art azimuthal resistivity tools.

In one aspect the present invention includes a downhole tool. Thedownhole hole tool includes a substantially cylindrical, electricallyconductive tool body including first and second longitudinally opposedends. The tool body further includes a blade deployed thereon, the bladebeing configured to extend outward from the tool body. At least onemagnetically permeable ring is deployed about the tool body between thefirst and second longitudinally opposed ends, and an AC voltage supplyis electrically connected to the first and second ends of the tool body.At least one current sensing electrode is deployed in an outer surfaceof the blade.

In another aspect, this invention includes a downhole tool. The toolincludes a substantially cylindrical, electrically conductive tool bodyincluding first and second longitudinally opposed ends. The tool bodyfurther includes a blade deployed thereon, the blade configured toextend outward from the tool body. At least one magnetically permeablering is deployed about the tool body between the first and secondlongitudinally opposed ends. An electrically conductive, rigid sleeve isdeployed about the magnetically permeable ring and an AC voltage supplyis electrically connected to the first end of the tool body and thesleeve. The sleeve provides an electrically conductive path exterior toan outer surface of the magnetically permeable ring. At least onecurrent sensing electrode is deployed in an outer surface of the blade.

In still another aspect, the invention includes a logging while drillingtool. The LWD tool includes a substantially cylindrical, electricallyconductive tool body including first and second longitudinally opposedends and a central region located between the first and second end. Atleast one blade is deployed on the central region of the tool body, theblade configured to extend outward from the tool body. Longitudinallyspaced first and second magnetically permeable rings are deployed aboutthe tool body such that the blade is located between the first andsecond magnetically permeable rings. At least one AC voltage supply isdisposed to provide an AC voltage difference between the central regionof the tool body and the longitudinally opposed ends of the tool body.At least one connecting conductor is deployed exterior to an outersurface of each of the magnetically permeable rings and at least onecurrent sensing electrode is deployed in an outer surface of blade.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiments disclosed may be readily utilized as abasis for modifying or designing other structures for carrying out thesame purposes of the present invention. It should also be realized bythose skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1A is a schematic representation of a portion of a prior artdownhole tool having a toroidal transformer deployed about a drillcollar.

FIG. 1B depicts an electrical circuit representation of the prior arttool shown on FIG. 1A.

FIG. 2 is a schematic representation of an offshore oil and/or gasdrilling platform utilizing an exemplary embodiment of the presentinvention.

FIG. 3A is a schematic representation of a portion of a downhole toolaccording to the present invention.

FIG. 3B depicts an electrical circuit representation of the tool shownon FIG. 3A.

FIG. 3C depicts an exemplary electrical circuit representation of thetool shown on FIG. 3A deployed in a subterranean borehole.

FIG. 4 depicts an exemplary embodiment of a downhole tool according tothe present invention.

FIG. 5A depicts, in cross section, a portion of the embodiment of FIG. 4showing an exemplary sleeve assembly deployed about the tool body.

FIG. 5B depicts, in cross section, a portion of the embodiment of FIG. 4showing an exemplary electrode deployed in a tool body.

FIG. 6 depicts an alternative embodiment of a downhole tool accordingthe present invention.

DETAILED DESCRIPTION

FIG. 1A schematically illustrates a wound toroidal core antenna 90deployed about a drill collar 80 as utilized in various prior artdownhole resistivity measurement tools. In a typical prior artapparatus, the wound toroidal core antenna 90 includes a toroidal core94 having multiple windings (N-turns) of insulated wire 96 wrappedthereabout. An AC voltage supply 92 is coupled to the ends of theinsulated wire 96. AC current passing through the windings induces amagnetic field in the toroidal core 94 circumferentially about the drillcollar 80. The circumferential magnetic field further induces an ACpotential difference in the drill collar 80 such that there is apotential difference between upper 82 and lower 84 sides thereof. Itwill be appreciated that terms used in this disclosure such as “upper”and “lower” are intended merely to show relative positionalrelationships in the described exemplary embodiments and are notlimiting of the invention in any way. As described briefly above in theBackground Section of this disclosure, the potential difference causesan electrical current to flow from one side of the drill collar (e.g.,upper side 82) through the borehole and subterranean formation to theother side of the drill collar (e.g., lower side 84). Such current flowthrough the formation (induced by the wound toroidal core antenna 90)and measurement thereof is the basis for certain prior art resistivitylogging techniques.

With further reference now to FIG. 1B, an electrical circuitrepresentation of the prior art arrangement shown in FIG. 1A isillustrated. As described in the prior art, the combination of the woundtoroidal core antenna 90 and drill collar 80, as shown in FIG. 1A, isessentially a N:1 stepdown transformer. Thus, when AC voltage source 92(providing V volts) is coupled to the N-turn primary winding 96, asecondary voltage with a magnitude of V/N is induced between the upper82 and lower 84 sides of the drill collar. The two sides 82 and 84 ofthe drill collar are approximately separate quasi-equipotential surfaceshaving a potential difference of V/N.

One possible alternative approach for providing a potential differencebetween upper and lower portions of a drill collar is to electricallyisolate the two portions of the drill collar. For example, an electricalinsulator may be deployed between the two portions of the drill collarand a voltage may be applied therebetween, for example via aconventional AC voltage supply. While such an approach is seeminglystraightforward, it is not likely to provide a viable solution. Ofparticular significance, a drill collar having first and second portionsseparated by an electrical insulator is not rigid enough for downholedrilling applications owing to the relatively poor mechanical propertiesof conventional electrical insulators (as compared, for example, tostainless steel). Thus, another alternative approach is required inorder to replace wound toroidal core antennas in certain downholeresistivity measurement tools.

Referring now to FIGS. 2 through 6, exemplary embodiments of the presentinvention are illustrated. FIG. 2 schematically illustrates oneexemplary embodiment of a logging while drilling tool 100 in use in anoffshore oil or gas drilling assembly, generally denoted 10. In FIG. 2,a semisubmersible drilling platform 12 is positioned over an oil or gasformation (not shown) disposed below the sea floor 16. A subsea conduit18 extends from deck 20 of platform 12 to a wellhead installation 22.The platform may include a derrick 26 and a hoisting apparatus 28 forraising and lowering the drill string 30, which, as shown, extends intoborehole 40 and includes a drill bit 32 and a measurement tool 100.Embodiments of LWD tool 100 include at least one magnetically permeablering 120 deployed about the tool body 110 (FIG. 3A). Exemplaryembodiments of LWD tool 100 may further optionally include (i) one ormore electrodes 140 configured to locally measure the current flowbetween the tool body 110 and the formation and (ii) an azimuth sensor180, which are advantageously longitudinally spaced from ring 120.Azimuth sensor 180 may include substantially any sensor that issensitive to its azimuth on the tool 100 (e.g., relative to high side),such as one or more accelerometers, magnetometers, and/or gyroscopes.Drill string 30 may further include a downhole drill motor, a mud pulsetelemetry system, and one or more other sensors, such as a nuclearlogging instrument, for sensing downhole characteristics of the boreholeand the surrounding formation.

It will be understood by those of ordinary skill in the art that thedeployment illustrated on FIG. 2 is merely exemplary for purposes ofdescribing the invention set forth herein. It will be further understoodthat the measurement tool 100 of the present invention is not limited touse with a semisubmersible platform 12 as illustrated on FIG. 2.Measurement tool 100 is equally well suited for use with any kind ofsubterranean drilling operation, either offshore or onshore.

In the embodiment shown on FIG. 2, azimuth sensor 180 is longitudinallyspaced from and deployed at substantially the same azimuthal(circumferential) position as the electrode 140. It will be appreciatedthat this invention is not limited to any particular layout(positioning) of the electrode(s) 140 and the azimuth sensor(s) 180 onthe tool 100. For example, in an alternative embodiment (not shown)electrode 140 and an azimuth sensor 180 may be deployed at substantiallythe same longitudinal position. Moreover, it will also be appreciatedthat this invention is not limited to any particular number ofelectrodes 140 and/or azimuth sensors 180. Furthermore, as described inmore detail below, certain exemplary methods of this invention do notrely on azimuth measurements or electrode measurements and hence do notrequire a downhole tool having an azimuth sensor or an electrode.

Referring now to FIG. 3A, a portion of one exemplary embodiment of LWDtool 100 from FIG. 2 is schematically illustrated. LWD tool 100 istypically a substantially cylindrical tool, being largely symmetricalabout longitudinal axis 70. In the exemplary embodiment shown,magnetically permeable ring 120 is deployed about (external to) asubstantially cylindrical conductive tool body 110 (e.g., a drillcollar). The tool body is configured for coupling to a drill string(e.g., drill string 30 on FIG. 2) and therefore typically, but notnecessarily, includes conventional threaded pin and box ends (notshown). An AC voltage supply 130 is electrically connected to the toolbody 110 on opposing sides 112 and 114 of the magnetically permeablering 120, with at least one connecting conductor 132 crossing theexterior (outer surface 122) of the ring 120. Such opposing sides arealso referred to herein as upper 112 and lower 114 sides for clarity ofexposition. It will be understood that the application is not limited bysuch terminology. Application of an AC current between the upper 112 andlower 114 sides of the tool body 110 induces a circumferential magneticflux in the ring 120. The magnetic flux in turn decreases the admittance(i.e., increases the impedance) between the upper 112 and lower 114sides of the tool body 110, which enables an AC potential difference tobe supported therebetween.

With further reference now to FIG. 3B, an electrical circuitrepresentation of the exemplary embodiment shown in FIG. 3A isillustrated. AC voltage source 130, having an AC voltage of V volts, iselectrically connected to upper 112 and lower 114 sides of the tool body110 with one of the conductors routed exterior to the magneticallypermeable ring 120 (as shown on FIG. 3A). As described above, theapplied AC voltage induces a magnetic flux in the ring 120, which inturn reduces the admittance Y of the tool body between the upper 112 andlower sides 114. Such a reduction in admittance Y enables the tool bodyto support an AC voltage difference of V volts between the upper 112 andlower 114 sides. While the admittance of the tool body 110 may besignificantly reduced, it is not reduced to zero. Therefore, a currentof V•Y will flow in the tool body 110 between the upper 112 and lower114 sides thereof.

It will be appreciated that in the configuration shown on FIG. 3A (andalso on FIGS. 4 and 6 as described in more detail below) themagnetically permeable ring 120 increases the inductance of the portionof the tool body 110 located internal to the ring 120, therebyconverting the otherwise conductive tool body 110 into an inductor. Theimpedance of such an inductor (the portion of the tool body locatedinternal to the ring 120) is substantially proportional to the both thefrequency of the AC voltage source 130 and the magnetic permeability ofthe ring 120. As described in more detail below (and as shown on FIG.5A), multiple rings, each having a high magnetic permeability, may beused to increase the inductance (and impedance) of the tool body andtherefore to reduce current flow in the tool body 110 between the upper112 and lower 114 sides.

With reference again to FIG. 3A, it is generally advantageous toconfigure LWD tool 100 so that the admittance between the upper 112 andlower 114 sides of the tool body is reduced (i.e., the impedance isincreased) as much as possible in order to decrease current flow in thetool body 110 between the upper 112 and lower 114 sides. This may beaccomplished, for example, by utilizing a magnetically permeable ring120 having a high magnetic permeability. While a magnetically permeablering 120 having substantially any suitable magnetic permeability may beutilized, one having a relative magnetic permeability of greater thanabout 10,000 is preferred. In such preferred embodiments, magneticallypermeable ring 120 may be fabricated from, for example, Supermalloy,Amorphous Alloy E, and Permalloy 80 (available from Magnetics, Inc.) andMetglas® 2714A and Metglas® 2605 (available from Allied-Signal).Increasing the number of magnetically permeable rings 120 deployed abouttool body 110 and the physical dimensions thereof (i.e., the radialthickness and longitudinal width of the rings 120) also tends todecrease the admittance between the opposing sides 112 and 114 of thetool body 110 by increasing the magnetic flux in the ring 120. However,it will be appreciated that in many applications there may be a tradeoffbetween a desire to further lower the admittance (and therefore to usemore and larger rings 120) and a desire for a relatively compact tool.Nevertheless, this invention is not limited to the number and size ofthe magnetically permeable rings 120 deployed about the tool body 110.

Embodiments of this invention may utilize substantially any suitablepower source 130. In one advantageous embodiment, power source 130provides an AC voltage perturbation having a frequency in the range offrom about 100 Hz to about 100 kHz and root mean square amplitude in therange of from about 50 mV to about 5 V. In general power source 130 isdeployed inside the tool body (to protect it from the severe boreholeenvironment) and is electrically connected to at least one conductor(e.g., conductor 132) routed about (exterior to) the magneticallypermeable ring 120. It will be appreciated that power source 130 is notlimited to a conventional sinusoidal AC voltage supply. Rather,substantially any power source providing substantially any AC voltagesignal may be utilized. For example, an AC voltage signal havingmultiple frequencies may be utilized (e.g., square wave, triangularwave, etc.). Moreover, in some embodiments it may be advantageous toutilize an AC voltage supply providing a plurality of distinctsinusoidal frequency components. In such an embodiment, the individualfrequency components may be utilized, for example, to infer resistivityvalues of different portions of the formation.

With further reference now to FIG. 3C, another electrical circuitrepresentation is illustrated in which LWD tool 100 is deployed in asubterranean borehole (such as borehole 40 shown on FIG. 2). Asdescribed above, AC voltage source 130 is electrically connected toupper 112 and lower 114 sides of the tool body, resulting in an ACpotential difference therebetween. Depending upon the electricalcharacteristics of the subterranean borehole (e.g., the formation andthe drilling fluid resistivities), such a potential difference may causeAC current to flow between the subterranean formation and the tool body.In the exemplary circuit representation shown, electrical current maytraverse three parallel paths between the upper 112 and lower 114 sidesof the tool body. A first path is through the tool body as representedby inductor L. As described above, it is generally desirable to minimizecurrent flow through the tool body by increasing the inductance thereof.Second and third paths are through the borehole (the drilling fluid) andthe formation. The resistors R_(b) and R_(f) represent borehole andformation impedances (which may also be referred to as apparentresistivities), respectively. To determine a local formation impedance(which is related to the formation resistivity), current flow betweenthe borehole and tool body 110 is measured. As described in more detailbelow, one or more electrodes (such as electrode 140 shown on FIG. 2)may be utilized to measure current flow between the borehole and thetool body 110.

Turning now to FIG. 4, one embodiment of a measurement tool 200according to principles of the present invention is illustrated.Measurement tool 200 includes at least one magnetically permeable ring220 deployed about a substantially cylindrical tool body 210. An ACvoltage supply (shown schematically at 230) is deployed in the tool body110 and electrically connected to upper 212 and lower 214 portions ofthe tool body. It will be appreciated that voltage supply 230 may bedeployed substantially anywhere in the tool 200 or elsewhere in thedrill string.

Measurement tool 200 further includes a rigid sleeve 250 deployed aboutthe magnetically permeable ring 220. The sleeve 250 is intended tophysically protect the ring 220 from the abrasive drilling environmentand collisions with the borehole wall. In the exemplary embodimentshown, sleeve 250 is threadably coupled to the lower portion 214 of thetool body 210, however, the invention is not limited in this regard.Physical protection for the ring 220 may be provided by substantiallyany additional and/or alternative means, such as via deployment of thering 220 in a recess in the tool body 210 (as shown for example on FIG.6). Sleeve 250 may also serve as an electrical conductor routed externalto the magnetically permeable ring 220 (exterior to the outer surface ofthe ring 220), thus serving the same purpose as the conductor 132 shownon FIG. 3A. While the AC voltage supply 230 is shown schematically onFIG. 4, it will be appreciated that the voltage supply 230 is typicallyhoused in the drill string (e.g., in tool body 210). Moreover, voltagesupply 230 is typically electrically connected to an internal surface(not shown) of the upper portion 212 of the tool body 210. An electricalconductor may extend through a hole in the tool body and connect to aninner surface 252 of sleeve 250, thus completing the circuit shown onFIG. 3B.

Turning now also to FIG. 5A, one exemplary embodiment of a sleeveassembly 270 is shown in greater detail. In the exemplary embodimentshown, sleeve assembly 270 includes a sleeve 250 having an electricalinsulating layer 224 (such as Teflon®g) interposed between inner surface252 and magnetically permeable rings 220A, 220B and 220C. It will beappreciated that insulating layer 224 is intended to prevent a shortcircuit between the sleeve 250 and the tool body 210 through one or moreof the rings 220A, 220B, and 220C, and therefore may alternativelyand/or additionally be interposed between the rings 220A, 220B, and 220Cand the tool body 210. The sleeve assembly 270 further includes at leastone conductor 254 that electrically connects the sleeve 250 to aterminal on AV voltage supply 230. Conductor 254 typically, although notnecessarily, includes a rigid member such as a pin or bolt welded orthreaded in place. Insulating material 258 serves to electricallyisolate the pin 254 from the tool body 210. AC voltage supply 230 (shownschematically in FIG. 5A) is electrically connected to pin 254 and theupper portion 212 of the tool body 210. The sleeve assembly 270 mayfurther include, for example, a washer 256 (or o-rings) interposedbetween the sleeve 250 and the tool body 210 to prevent contamination,such as drilling fluid and other downhole debris from entering annularcavity 228. Washer 256 may be fabricated, for example, from aPolyetheretherketone, such as PEEK (available from Victrex Corporation,Lancashire, UK). Annular cavity 228 may further be filled with aninsulating material, such as an injectable silicon rubber, to providefurther electrical insulation between sleeve 250 and tool body 210. Sucha filler material may also provide vibration isolation for themagnetically permeable rings 220A, 220B, and 220C.

With reference again to FIG. 4, exemplary embodiments of measurementtool 200 further include one or more electrodes 240 deployed in andelectrically isolated from the lower portion 214 of the tool body 210.Such electrodes may alternatively, or additionally, be deployed in theupper portion 212 of the tool body 210, and are intended to provide asegregated path for current flow between the formation and the tool body210. The formation resistivity in a region of the formation generallyopposing the electrode 240 may be determined via measurement of the ACcurrent in the electrode 240. The apparent formation resistivity isinversely proportional to the current measured at the electrode 240.Assuming that the tool body is an equipotential surface, the apparentformation resistivity may be approximated mathematically by theequation: R_(f)=V/I, where V represents the voltage between the upper212 and lower 214 sides of the tool body and I represents the measuredcurrent. It will be appreciated that various corrections may be appliedto the apparent formation resistivity to compensate, for example, forborehole resistivity, electromagnetic skin effect, and geometric factorsthat are known to influence the measured current at electrode 240.

With reference now to FIGS. 4 and 5B, one exemplary embodiment ofelectrode 240 is shown in more detail. Electrode 240 is mounted in aninsulating material 244 such as a Viton® rubber (DuPont® de Nemours,Wilmington, Del.). Insulating material 244 serves to electricallyisolate the outer surface (face 243 of the electrode 240 from the outersurface of the tool body 210. A neck portion 241 of the electrode may bethreaded, for example, to the tool body 210 and thus electricallyconnected thereto.

In one embodiment, the electrode 240 is generally circular in shape(i.e., having a circular periphery), although the invention is notlimited in this regard. Moreover, the electrode face 243 may include agenerally cylindrical curvature to conform to the outer surface of thetool body 210 (e.g., to protect it from the borehole environment).Alternatively, the electrode 240 may include a flat face 243 that isslightly recessed in the tool body. Again the invention is not limitedin regard to the shape of the electrode 240. In general the electrode240 spans only a small fraction of the total circumference of the toolbody 210 and thus may provide azimuthally sensitive resistivitymeasurements. Moreover, the electrode 240 also has a vertical extentthat is a small fraction of the length of the tool 200 and thus mayprovide for axially sensitive (along the axis of the borehole)resistivity measurements. As such, in certain advantageous embodiments,the face 243 of the electrode 240 may have a diameter in the range offrom about 1 to about 4 centimeters, which is large enough to providesufficient signal (current) and small enough to provide the desiredvertical and azimuthal resolution. However, the invention is not limitedby the size of the electrode 240.

With continued reference to FIGS. 4 and 5, a conventional currentmeasuring transformer 262 may be deployed about neck portion 241 andutilized to measure the AC current in the electrode 240. Such anarrangement advantageously functions as a very low impedance ammeter. Itwill be appreciated that substantially any other suitable arrangementmay be utilized to measure the AC current in the electrode 240. Forexample, a current sampling resistor (preferably having a resistancesignificantly less than the sum of the formation and boreholeresistances) may be utilized in conjunction with a conventionalvoltmeter. Alternatively, a Hall effect device or other similarnon-contact measurement may be utilized to infer the current flowing inthe electrode via measurement of a magnetic field. In still anotheralternative embodiment, a conventional operational amplifier and afeedback resistor may be utilized. Nevertheless, it will be appreciatedthat this invention is not limited by any particular technique utilizedto measure the electrical current in the electrode(s).

In use, measurement tool 200 is typically coupled to a drill string androtated in a borehole. The AC current may be, for example, continuouslymeasured at electrode 240 and averaged over some predetermined samplinginterval (e.g., 10 milliseconds). The duration of each sampling intervalis preferably significantly less than the period of the tool rotation inthe borehole (e.g., the sampling interval may be about 10 milliseconds,as stated above, while the rotational period of the tool may be about0.5 seconds). Meanwhile, an azimuth sensor (such as azimuth sensor 180shown on FIG. 2) measures the azimuth at the electrode 240, as the toolrotates in the borehole. The average current in each sampling intervalmay then be utilized to calculate a local formation resistivity at aparticular azimuth. The azimuth is preferably measured at each samplinginterval, or often enough so that the azimuth of the tool may bedetermined for each resistivity value, although the invention is notlimited in this regard.

It will be appreciated that such azimuthally sensitive resistivitymeasurements may be utilized to form a two-dimensional image of theformation resistivity versus the azimuthal position in the borehole andthe well depth. To form a two dimensional image (azimuthal positionversus well depth), resistivity measurements may be acquired at aplurality of well depths using substantially any suitable procedure. Forexample, in one exemplary embodiment, azimuthally sensitive resistivitydata may be acquired substantially continuously as described aboveduring at least a portion of a drilling operation. Such resistivity datamay be grouped by time (e.g., in 10 second intervals) with each groupindicative of a single well depth. At a drilling rate of about 60 feetper hour, a 10 second interval represents about a two-inch depthinterval. In certain imaging applications it may be advantageous toutilize conventional false color rendering or gray-scale rendering ofthe resistivity measurements. It will be appreciated that this inventionis not limited to any particular sampling intervals and/or time periods.Nor is this invention limited by the description of the above exemplaryembodiments.

It will be appreciated that exemplary embodiments of measurement tool200 may include a plurality of electrodes 240 deployed about theperiphery of the tool 200. Such embodiments may advantageously enableazimuthally sensitive resistivity measurements to be made about thecircumference of the borehole without rotation of the drill string.Moreover, when used with a rotating drill string, such embodiments mayadvantageously provide for redundancy as well as reduced system noiseaccomplished via data averaging at each of the electrodes at eachazimuthal position about the circumference of the borehole.

Exemplary embodiments of measurement tool 200 may also include two ormore electrodes 240 deployed at substantially the same azimuthalposition but longitudinally offset from one another (e.g., shown aselectrodes 340A, 340B, and 340C on FIG. 6 described in more detailbelow). In such embodiments, the electrode(s) that are located fartherfrom the magnetic rings 220 are expected to provide resistivitymeasurements that tend to be respectively deeper into the subterraneanformation than electrode(s) that are located nearer to the rings 220.Such longitudinal electrode spacing may then advantageously provide forvertically resolved resistivity at multiple depths of investigation.Again, as stated above, this invention is not limited to any particularelectrode spacing.

Moreover, it will further be appreciated that this invention is notlimited to the use of an electrode or an azimuth sensor. For example,exemplary embodiments of this invention may include a conventionaltoroidal receiver deployed about the upper 212 and/or the lower 214portion of the tool body 210. In such embodiments, the combination ofthe magnetic ring(s) and the voltage supply coupled to upper and lowerends of the tool body take the place of the conventional toroidaltransmitter. The toroidal receiver may be utilized to measure currentflow in the tool body and hence to determine a non-azimuthally sensitiveformation resistivity.

Referring now to FIG. 6, another alternative embodiment of a measurementtool 300 according to the principles of this invention is shown. In theexemplary embodiment shown, measurement tool 300 is configured as adownhole steering tool, such as a three-dimensional rotary steerabletool. Measurement tool 300 includes a tool body 310 and at least oneblade 315 deployed, for example, in a recess 313 in the tool body 310.The tool body 310 is configured to be substantially non-rotating (withrespect to the borehole) and is typically deployed about a rotatingshaft 305, which transfers torque to a drill bit (e.g., coupled torotating shaft 305 at drill bit receptacle 307). Tool 300 may thusincorporate one or more sealing assemblies 390 and bearing assemblies392 deployed between shaft 305 and tool body 310. The blade(s) 315 areconfigured to extend outward from the tool body 310 (in a directionsubstantially perpendicular to the longitudinal axis of the tool), forexample, into contact with a borehole wall.

With continued reference to FIG. 6, measurement tool 300 includes firstand second longitudinally spaced magnetically permeable rings 320A and320B deployed about tool body 310. First and second sleeves 350A and350B are deployed about the corresponding first and second magneticallypermeable rings 320A and 320B to both protect the rings 320A and 320Band to provide an electrically conductive path external thereto asdescribed above with respect to FIG. 4. A first AC voltage supply (shownschematically at 330A) is coupled to sleeve 350A and the tool body 310while a second AC voltage supply (shown schematically at 330B) iscoupled to sleeve 350B and the tool body 310. In the embodiment shown,AC voltage supplies 330A and 330B are configured with opposingpolarities, for example, such that sleeve 350A is coupled to thenegative terminal of voltage supply 330A, while sleeve 350B is coupledto the positive terminal of voltage supply 330B or visa versa. Such aconfiguration including first and second magnetically permeable rings320A and 320B and corresponding voltage supplies 330A and 330B havingopposing polarities tends to advantageously provide a highlyequipotential surface in the region 317 of the tool body 310 locatedbetween the rings 320A and 320B. While first and second voltagessupplies are shown schematically in FIG. 6, the artisan of ordinaryskill will readily recognize that a single voltage supply may also beused. For example, a first terminal of the voltage supply may beelectrically connected to the tool body at a location between themagnetically permeable rings and a second terminal of the voltage supplymay be electrically connected to each of the ends of the tool body.

Exemplary embodiments of measurement tool 300 further include aplurality of electrodes 340A, 340B, 340C, and 341. In the exemplaryembodiment shown, at least one blade 315 (e.g., out of three blades inan exemplary rotary steerable tool embodiment) includes threelongitudinally spaced electrodes 340A, 340B, and 340C. As describedabove, such longitudinal spacing of the electrodes 340A, 340B, and 340Cmay advantageously enable vertically resolved resistivity measurementsto be acquired at multiple depths of investigation. Moreover, deploymentof the electrodes 340A, 340B, and 340C on a stabilizer or steering toolblade 315 may advantageously reduce the effects of the borehole (e.g.,the drilling fluid resistivity) on the formation resistivitymeasurements (e.g., by enabling the electrodes 340A, 340B, and 340C tobe located in close proximity to or even in contact with the boreholewall during measurement). It will be appreciated that additionalelectrodes 341 may also be located directly on the tool body 310 (e.g.,between the blades 315). Since tool body 310 is configured to besubstantially non-rotating relative to the borehole in the embodimentshown, it may be advantageous to include a plurality of electrodes aboutthe periphery of the tool to acquire azimuthally sensitive resistivitymeasurements. For example, such electrodes may be deployed on each blade(e.g., each of three or four blades on a typical steering tool) and/orat one or more azimuthal positions on the tool body between blades.

It will be understood that while not shown in the FIGS. 2 through 6,embodiments of this invention may include an electronic controller. Sucha controller may include, for example, a programmable processor, such asa microprocessor or a microcontroller, volatile or non-volatile memory,and/or a data storage device. The controller may also includeprocessor-readable or computer-readable program code embodying logic,including instructions for controlling the function of the AC voltagesupply(ies) and/or measuring AC current in the electrode(s). A suitableprocessor may be further utilized, for example, to determine formationresistivity based on measured electrode current. Such resistivity valuesmay be stored in memory (e.g., in the controller) and/or transmitted tothe surface.

It will be appreciated that the above described AC voltage supplies(e.g., voltage supplies 330A and 330B shown in FIG. 6) may beincorporated into a suitable controller. Such a controller may include,for example, conventional electrical drive electronics for applying avoltage waveform (e.g., having a plurality of distinct sinusoidalcomponents) to the tool body. The controller may further include acurrent limiting mechanism for preventing excessive currents in theevent that one or more of the magnetic permeable rings are damaged orfractured during operations. A suitable controller may also includereceiving electronics, such as a variable gain amplifier for amplifyingthe electrode current signals. The receiving electronics may alsoinclude various filters (e.g., low and/or high pass filters),rectifiers, multiplexers, and other circuit components for processingelectrode current signals.

A suitable controller may also optionally include other controllablecomponents, such as sensors, data storage devices, power supplies,timers, and the like. The controller may also be disposed to be inelectronic communication with various sensors and/or probes formonitoring physical parameters of the borehole, such as a depthdetection sensor and/or an accelerometer, gyro or magnetometer to detectazimuth and inclination. A suitable controller may also optionallycommunicate with other instruments in the drill string, such astelemetry systems that communicate with the surface. The artisan ofordinary skill will readily recognize that a suitable controller may bedeployed substantially anywhere within the measurement tool or atanother suitable location in the drill string.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalternations can be made to the embodiments set forth herein withoutdeparting from the spirit and scope of the invention as defined by theappended claims.

1. A logging while drilling tool comprising: a substantiallycylindrical, electrically conductive tool body including first andsecond longitudinally opposed ends, the tool body including a bladedeployed thereon, the blade configured to extend outward from the toolbody; at least one magnetically permeable ring deployed about the toolbody between the first and second longitudinally opposed ends thereof;an AC voltage supply electrically connected to the first and second endsof the tool body with at least one connecting conductor deployedexterior to an outer surface of the magnetically permeable ring; and atleast one current sensing electrode deployed in an outer surface of theblade.
 2. The logging while drilling tool of claim 1, wherein themagnetically permeable ring has a relative permeability of greater thanabout 10,000.
 3. The logging while drilling tool of claim 1, wherein theat least one connecting conductor comprises a rigid member deployedabout the magnetically permeable ring, the voltage supply beingconnected to the second end of the tool body via the rigid member, themember providing an electrically conductive path exterior to an outersurface of the magnetically permeable ring.
 4. The logging whiledrilling tool of claim 1, wherein the magnetically permeable ring isdeployed in a recess in an outer surface of the tool body.
 5. Thelogging while drilling tool of claim 1, comprising a plurality ofcurrent sensing electrodes longitudinally spaced along the blade.
 6. Thelogging while drilling tool of claim 1, comprising a plurality ofcircumferentially spaced blades, each of the blades including at leastone current sensing electrode.
 7. The logging while drilling tool ofclaim 1, comprising an additional current sensing electrode deployed inan outer surface of the tool body.
 8. The logging while drilling tool ofclaim 1, comprising a plurality of current sensing electrodescircumferentially spaced about the tool body.
 9. The logging whiledrilling tool of claim 1, wherein: a neck portion of the current sensingelectrode is mechanically and electrically connected to the blade; andan outer surface of the current sensing electrode is electricallyisolated from the outer surface of the blade.
 10. The logging whiledrilling tool of claim 9, further comprising a current measuringtransformer deployed about the neck portion of the current sensingelectrode.
 11. A logging while drilling tool comprising: a substantiallycylindrical, electrically conductive tool body including first andsecond longitudinally opposed ends, the tool body including a bladedeployed thereon, the blade configured to extend outward from the toolbody; at least one magnetically permeable ring deployed about the toolbody between the first and second longitudinally opposed ends thereof;an electrically conductive, rigid sleeve deployed about the magneticallypermeable ring; an AC voltage supply electrically connected to the firstend of the tool body and the sleeve, the sleeve providing anelectrically conductive path exterior to an outer surface of themagnetically permeable ring; and at least one current sensing electrodedeployed in an outer surface of the blade.
 12. The logging whiledrilling tool of claim 11, wherein the sleeve is threadably coupled withthe tool body.
 13. The logging while drilling tool of claim 11, furthercomprising an insulating layer deployed between the magneticallypermeable ring and at least one of the tool body and the sleeve.
 14. Thelogging while drilling tool of claim 11 wherein the magneticallypermeable ring is deployed in the sleeve.
 15. A logging while drillingtool comprising: a substantially cylindrical, electrically conductivetool body including first and second longitudinally opposed ends and acentral region located between the first and second ends; at least oneblade deployed on the central region of the tool body, the bladeconfigured to extend outward from the tool body; longitudinally spacedfirst and second magnetically permeable rings deployed about the toolbody such that the blade is located between the first and secondmagnetically permeable rings; at least one AC voltage supply disposed toprovide an AC voltage difference between the central region of the toolbody and the longitudinally opposed ends of the tool body; at least oneconnecting conductor deployed exterior to an outer surface of each ofthe magnetically permeable rings; and at least one current sensingelectrode deployed in an outer surface of blade.
 16. The logging whiledrilling tool of claim 15, wherein a first terminal of the voltagesupply is electrically connected to the central region of the tool bodyand a second terminal of the voltage supply is electrically connected toeach of the first and second ends of the tool body.
 17. The loggingwhile drilling tool of claim 15, comprising first and second AC voltagesupplies, the first AC voltage supply electrically connected to thefirst end and the central region of the tool body with at least oneconnecting conductor between the first end and the central region of thetool body deployed external the first magnetically permeable ring, thesecond AC voltage supply electrically connected to the second end andthe central region of the tool body with at least one connectingconductor between the second end and the central region of the tool bodydeployed external to the second magnetically permeable ring.
 18. Thelogging while drilling tool of claim 15, wherein the connectingconductors comprise corresponding first and second rigid sleevesdeployed about the first and second magnetically permeable rings, thesleeves providing an electrically conductive path exterior to each ofthe magnetically permeable rings.
 19. The logging while drilling tool ofclaim 15, wherein the first and second magnetically permeable rings eachhave a relative permeability of greater than about 10,000.
 20. Thelogging while drilling tool of claim 15, wherein: a neck portion of thecurrent sensing electrode is mechanically and electrically connected tothe blade; and an outer surface of the current sensing electrode iselectrically isolated from the outer surface of the blade.